Whether an urban, suburban, or rural gardener, you should make no assumptions about the depth and openness of the soil at your disposal. Dig a test hole. If you find less than 2 unfortunate feet of open earth before hitting an impermeable obstacle such as rock or gravel, not much water storage can occur and the only use this book will hold for you is to guide your move to a more likely gardening location or encourage the house hunter to seek further. Of course, you can still garden quite successfully on thin soil in the conventional, irrigated manner. Growing Vegetables West of the Cascades will be an excellent guide for this type of situation.
Eliminating Plowpan
Deep though the soil may be, any restriction of root expansion greatly limits the ability of plants to aggressively find water. A compacted subsoil or even a thin compressed layer such as plowpan may function as such a barrier. Though moisture will still rise slowly by capillarity and recharge soil above plowpan, plants obtain much more water by rooting into unoccupied, damp soil. Soils close to rivers or on floodplains may appear loose and infinitely deep but may hide subsoil streaks of droughty gravel that effectively stops root growth. Some of these conditions are correctable and some are not.
Plowpan is very commonly encountered by homesteaders on farm soils and may be found in suburbia too, but fortunately it is the easiest obstacle to remedy. Traditionally, American croplands have been tilled with the moldboard plow. As this implement first cuts and then flips a 6- or 7-inch-deep slice of soil over, the sole--the part supporting the plow's weight--presses heavily on the earth about 7 inches below the surface. With each subsequent plowing the plow sole rides at the same 7-inch depth and an even more compacted layer develops. Once formed plowpan prevents the crop from rooting into the subsoil. Since winter rains leach nutrients from the topsoil and deposit them in the subsoil, plowpan prevents access to these nutrients and effectively impoverishes the field. So wise farmers periodically use a subsoil plow to fracture the pan.
Plowpan can seem as firm as a rammed-earth house; once established, it can last a long, long time. My own garden land is part of what was once an old wheat farm, one of the first homesteads of the Oregon Territory. From about 1860 through the 1930s, the field produced small grains. After wheat became unprofitable, probably because of changing market conditions and soil exhaustion, the field became an unplowed pasture. Then in the 1970s it grew daffodil bulbs, occasioning more plowing. All through the '80s my soil again rested under grass. In 1987, when I began using the land, there was still a 2-inch-thick, very hard layer starting about 7 inches down. Below 9 inches the open earth is soft as butter as far as I've ever dug.
On a garden-sized plot, plowpan or compacted subsoil is easily opened with a spading fork or a very sharp common shovel. After normal rotary tilling, either tool can fairly easily be wiggled 12 inches into the earth and small bites of plowpan loosened. Once this laborious chore is accomplished the first time, deep tillage will be far easier. In fact, it becomes so easy that I've been looking for a custom-made fork with longer tines.
Curing Clayey Soils
In humid climates like ours, sandy soils may seem very open and friable on the surface but frequently hold some unpleasant subsoil surprises. Over geologic time spans, mineral grains are slowly destroyed by weak soil acids and clay is formed from the breakdown products. Then heavy winter rainfall transports these minuscule clay particles deeper into the earth, where they concentrate. It is not unusual to find a sandy topsoil underlaid with a dense, cement-like, clayey sand subsoil extending down several feet. If very impervious, a thick, dense deposition like this may be called hardpan.
The spading fork cannot cure this condition as simply as it can eliminate thin plowpan. Here is one situation where, if I had a neighbor with a large tractor and subsoil plow, I'd hire him to fracture my land 3 or 4 feet deep. Painstakingly double or even triple digging will also loosen this layer. Another possible strategy for a smaller garden would be to rent a gasoline-powered posthole auger, spread manure or compost an inch or two thick, and then bore numerous, almost adjoining holes 4 feet deep all over the garden.
Clayey subsoil can supply surprisingly larger amounts of moisture than the granular sandy surface might imply, but only if the earth is opened deeply and becomes more accessible to root growth. Fortunately, once root development increases at greater depths, the organic matter content and accessibility of this clayey layer can be maintained through intelligent green manuring, postponing for years the need to subsoil again. Green manuring is discussed in detail shortly.
Other sites may have gooey, very fine clay topsoils, almost inevitably with gooey, very fine clay subsoils as well. Though incorporation of extraordinarily large quantities of organic matter can turn the top few inches into something that behaves a little like loam, it is quite impractical to work in humus to a depth of 4 or 5 feet. Root development will still be limited to the surface layer. Very fine clays don't make likely dry gardens.
Not all clay soils are "fine clay soils," totally compacted and airless. For example, on the gentler slopes of the geologic old Cascades, those 50-million-year-old black basalts that form the Cascades foothills and appear in other places throughout the maritime Northwest, a deep, friable, red clay soil called (in Oregon) Jori often forms. Jori clays can be 6 to 8 feet deep and are sufficiently porous and well drained to have been used for highly productive orchard crops. Water-wise gardeners can do wonders with Joris and other similar soils, though clays never grow the best root crops.
Spotting a Likely Site
Observing the condition of wild plants can reveal a good site to garden without much irrigation. Where Himalaya or Evergreen blackberries grow 2 feet tall and produce small, dull-tasting fruit, there is not much available soil moisture. Where they grow 6 feet tall and the berries are sweet and good sized, there is deep, open soil. When the berry vines are 8 or more feet tall and the fruits are especially huge, usually there is both deep, loose soil and a higher than usual amount of fertility.
Other native vegetation can also reveal a lot about soil moisture reserves. For years I wondered at the short leaders and sad appearance of Douglas fir in the vicinity of Yelm, Washington. Were they due to extreme soil infertility? Then I learned that conifer trees respond more to summertime soil moisture than to fertility. I obtained a soil survey of Thurston County and discovered that much of that area was very sandy with gravelly subsoil. Eureka!
The Soil Conservation Service (SCS), a U.S. Government agency, has probably put a soil auger into your very land or a plot close by. Its tests have been correlated and mapped; the soils underlying the maritime Northwest have been named and categorized by texture, depth, and ability to provide available moisture. The maps are precise and detailed enough to approximately locate a city or suburban lot. In 1987, when I was in the market for a new homestead, I first went to my county SCS office, mapped out locations where the soil was suitable, and then went hunting. Most counties have their own office.
Using Humus to Increase Soil Moisture
Maintaining topsoil humus content in the 4 to 5 percent range is vital to plant health, vital to growing more nutritious food, and essential to bringing the soil into that state of easy workability and cooperation known as good tilth. Humus is a spongy substance capable of holding several times more available moisture than clay. There are also new synthetic, long-lasting soil amendments that hold and release even more moisture than humus. Garden books frequently recommend tilling in extraordinarily large amounts of organic matter to increase a soil's water-holding capacity in the top few inches.
Humus can improve many aspects of soil but will not reduce a garden's overall need for irrigation, because it is simply not practical to maintain sufficient humus deeply enough. Rotary tilling only blends amendments into the top 6 or 7 inches of soil. Rigorous double digging by actually trenching out 12 inches and then spading up the next foot theoretically allows one to mix in significant amounts of organic matter to nearly 24 inches. But plants can use water from far deeper than that. Let's realistically consider how much soil moisture reserves might be increased by double digging and incorporating large quantities of organic matter.
A healthy topsoil organic matter level in our climate is about 4 percent. This rapidly declines to less than 0.5 percent in the subsoil. Suppose inches-thick layers of compost were spread and, by double digging, the organic matter content of a very sandy soil were amended to 10 percent down to 2 feet. If that soil contained little clay, its water-holding ability in the top 2 feet could be doubled. Referring to the chart "Available Moisture" in Chapter 2, we see that sandy soil can release up to 1 inch of water per foot. By dint of massive amendment we might add 1 inch of available moisture per foot of soil to the reserve. That's 2 extra inches of water, enough to increase the time an ordinary garden can last between heavy irrigations by a week or 10 days.
If the soil in question were a silty clay, it would naturally make 2 1/2 inches available per foot. A massive humus amendment would increase that to 3 1/2 inches in the top foot or two, relatively not as much benefit as in sandy soil. And I seriously doubt that many gardeners would be willing to thoroughly double dig to an honest 24 inches.
Trying to maintain organic matter levels above 10 percent is an almost self-defeating process. The higher the humus level gets, the more rapidly organic matter tends to decay. Finding or making enough well-finished compost to cover the garden several inches deep (what it takes to lift humus levels to 10 percent) is enough of a job. Double digging just as much more into the second foot is even more effort. But having to repeat that chore every year or two becomes downright discouraging. No, either your soil naturally holds enough moisture to permit dry gardening, or it doesn't.
Keeping the Subsoil Open with Green Manuring
When roots decay, fresh organic matter and large, long-lasting passageways can be left deep in the soil, allowing easier air movement and facilitating entry of other roots. But no cover crop that I am aware of will effectively penetrate firm plowpan or other resistant physical obstacles. Such a barrier forces all plants to root almost exclusively in the topsoil. However, once the subsoil has been mechanically fractured the first time, and if recompaction is avoided by shunning heavy tractors and other machinery, green manure crops can maintain the openness of the subsoil.
To accomplish this, correct green manure species selection is essential. Lawn grasses tend to be shallow rooting, while most regionally adapted pasture grasses can reach down about 3 feet at best. However, orchard grass (called coltsfoot in English farming books) will grow down 4 or more feet while leaving a massive amount of decaying organic matter in the subsoil after the sod is tilled in. Sweet clover, a biennial legume that sprouts one spring then winters over to bloom the next summer, may go down 8 feet. Red clover, a perennial species, may thickly invade the top 5 feet. Other useful subsoil busters include densely sown Umbelliferae such as carrots, parsley, and parsnip. The chicory family also makes very large and penetrating taproots.
Though seed for wild chicory is hard to obtain, cheap varieties of endive (a semicivilized relative) are easily available. And several pounds of your own excellent parsley or parsnip seed can be easily produced by letting about 10 row feet of overwintering roots form seed. Orchard grass and red clover can be had quite inexpensively at many farm supply stores. Sweet clover is not currently grown by our region's farmers and so can only be found by mail from Johnny's Selected Seeds (see Chapter 5 for their address). Poppy seed used for cooking will often sprout. Sown densely in October, it forms a thick carpet of frilly spring greens underlaid with countless massive taproots that decompose very rapidly if the plants are tilled in in April before flower stalks begin to appear. Beware if using poppies as a green manure crop: be sure to till them in early to avoid trouble with the DEA or other authorities.
For country gardeners, the best rotations include several years of perennial grass-legume-herb mixtures to maintain the openness of the subsoil followed by a few years of vegetables and then back (see Newman Turner's book in more reading). I plan my own garden this way. In October, after a few inches of rain has softened the earth, I spread 50 pounds of agricultural lime per 1,000 square feet and break the thick pasture sod covering next year's garden plot by shallow rotary tilling. Early the next spring I broadcast a concoction I call "complete organic fertilizer" (see Growing Vegetables West of the Cascades or the Territorial Seed Company Catalog), till again after the soil dries down a bit, and then use a spading fork to open the subsoil before making a seedbed. The first time around, I had to break the century-old plowpan--forking compacted earth a foot deep is a lot of work. In subsequent rotations it is much much easier.
For a couple of years, vegetables will grow vigorously on this new ground supported only with a complete organic fertilizer. But vegetable gardening makes humus levels decline rapidly. So every few years I start a new garden on another plot and replant the old garden to green manures. I never remove vegetation during the long rebuilding under green manures, but merely mow it once or twice a year and allow the organic matter content of the soil to redevelop. If there ever were a place where chemical fertilizers might be appropriate around a garden, it would be to affordably enhance the growth of biomass during green manuring.
Were I a serious city vegetable gardener, I'd consider growing vegetables in the front yard for a few years and then switching to the back yard. Having lots of space, as I do now, I keep three or four garden plots available, one in vegetables and the others restoring their organic matter content under grass.
Mulching
Gardening under a permanent thick mulch of crude organic matter is recommended by Ruth Stout (see the listing for her book in More Reading) and her disciples as a surefire way to drought-proof gardens while eliminating virtually any need for tillage, weeding, and fertilizing. I have attempted the method in both Southern California and western Oregon--with disastrous results in both locations. What follows in this section is addressed to gardeners who have already read glowing reports about mulching.
Permanent mulching with vegetation actually does not reduce summertime moisture loss any better than mulching with dry soil, sometimes called "dust mulching." True, while the surface layer stays moist, water will steadily be wicked up by capillarity and be evaporated from the soil's surface. If frequent light sprinkling keeps the surface perpetually moist, subsoil moisture loss can occur all summer, so unmulched soil could eventually become desiccated many feet deep. However, capillary movement only happens when soil is damp. Once even a thin layer of soil has become quite dry it almost completely prevents any further movement. West of the Cascades, this happens all by itself in late spring. One hot, sunny day follows another, and soon the earth's surface seems parched.
Unfortunately, by the time a dusty layer forms, quite a bit of soil water may have risen from the depths and been lost. The gardener can significantly reduce spring moisture loss by frequently hoeing weeds until the top inch or two of earth is dry and powdery. This effort will probably be necessary in any case, because weeds will germinate prolifically until the surface layer is sufficiently desiccated. On the off chance it should rain hard during summer, it is very wise to again hoe a few times to rapidly restore the dust mulch. If hand cultivation seems very hard work, I suggest you learn to sharpen your hoe.
A mulch of dry hay, grass clippings, leaves, and the like will also retard rapid surface evaporation. Gardeners think mulching prevents moisture loss better than bare earth because under mulch the soil stays damp right to the surface. However, dig down 4 to 6 inches under a dust mulch and the earth is just as damp as under hay. And, soil moisture studies have proved that overall moisture loss using vegetation mulch slightly exceeds loss under a dust mulch.
West of the Cascades, the question of which method is superior is a bit complex, with pros and cons on both sides. Without a long winter freeze to set populations back, permanent thick mulch quickly breeds so many slugs, earwhigs, and sowbugs that it cannot be maintained for more than one year before vegetable gardening becomes very difficult. Laying down a fairly thin mulch in June after the soil has warmed up well, raking up what remains of the mulch early the next spring, and composting it prevents destructive insect population levels from developing while simultaneously reducing surface compaction by winter rains and beneficially enhancing the survival and multiplication of earthworms. But a thin mulch also enhances the summer germination of weed seeds without being thick enough to suppress their emergence. And any mulch, even a thin one, makes hoeing virtually impossible, while hand weeding through mulch is tedious.
Mulch has some unqualified pluses in hotter climates. Most of the organic matter in soil and consequently most of the available nitrogen is found in the surface few inches. Levels of other mineral nutrients are usually two or three times as high in the topsoil as well. However, if the surface few inches of soil becomes completely desiccated, no root activity will occur there and the plants are forced to feed deeper, in soil far less fertile. Keeping the topsoil damp does greatly improve the growth of some shallow-feeding species such as lettuce and radishes. But with our climate's cool nights, most vegetables need the soil as warm as possible, and the cooling effect of mulch can be as much a hindrance as a help. I've tried mulching quite a few species while dry gardening and found little or no improvement in plant growth with most of them. Probably, the enhancement of nutrition compensates for the harm from lowering soil temperature. Fertigation is better all around.
Windbreaks
Plants transpire more moisture when the sun shines, when temperatures are high, and when the wind blows; it is just like drying laundry. Windbreaks also help the garden grow in winter by increasing temperature. Many other garden books discuss windbreaks, and I conclude that I have a better use for the small amount of words my publisher allows me than to repeat this data; Binda Colebrook's [i]Winter Gardening in the Maritime Northwest[i] (Sasquatch Books, 1989) is especially good on this topic.
Fertilizing, Fertigating and Foliar Spraying
In our heavily leached region almost no soil is naturally rich, while fertilizers, manures, and potent composts mainly improve the topsoil. But the water-wise gardener must get nutrition down deep, where the soil stays damp through the summer.
If plants with enough remaining elbow room stop growing in summer and begin to appear gnarly, it is just as likely due to lack of nutrition as lack of water. Several things can be done to limit or prevent midsummer stunting. First, before sowing or transplanting large species like tomato, squash or big brassicas, dig out a small pit about 12 inches deep and below that blend in a handful or two of organic fertilizer. Then fill the hole back in. This double-digging process places concentrated fertility mixed 18 to 24 inches below the seeds or seedlings.
Foliar feeding is another water-wise technique that keeps plants growing through the summer. Soluble nutrients sprayed on plant leaves are rapidly taken into the vascular system. Unfortunately, dilute nutrient solutions that won't burn leaves only provoke a strong growth response for 3 to 5 days. Optimally, foliar nutrition must be applied weekly or even more frequently. To efficiently spray a garden larger than a few hundred square feet, I suggest buying an industrial-grade, 3-gallon backpack sprayer with a side-handle pump. Approximate cost as of this writing was $80. The store that sells it (probably a farm supply store) will also support you with a complete assortment of inexpensive nozzles that can vary the rate of emission and the spray pattern. High-quality equipment like this outlasts many, many cheaper and smaller sprayers designed for the consumer market, and replacement parts are also available. Keep in mind that consumer merchandise is designed to be consumed; stuff made for farming is built to last.